1. Field of the Invention
The present invention relates to a novel process or the removal of non-ferrous metals, such as copper, tin, zinc or the like, from solid ferrous scrap.
2. Related Art Statement
At the present, in North America, approximately 40% of total steel production depends on electric arc furnaces. Because of a number of advantages of the electric arc furnaces in comparison with basic oxygen processes, increasing trend in the production of electric-furnace steel would still continue even in the next centrury.
In terms of steel specifications and product quality, however, the electric-furnace steel has not always been without problems. Needless to say, the electric-furnace steelmaking relies on the supplies of ferrous scrap. At the present, the electric-furnace steelmakers produce primarily bars and structural shapes, while thin plate or sheet steel for deep drawing cannot be made via electric arc furnaces, because of relatively high concentrations of copper in the ferrous scrap. It has been well known that copper in steel at concentrations higher than 0.1% by weight causes hot shortness or embrittlement, which results in production problems especially during continuous casting of molten steel and mechanical working of solidified steel. The electric arc furnaces are not able to produce a low copper steel from a high copper scrap. Hence, the high copper scrap has not been utilized in electric furnace steelmaking for producing high-grade steel.
In general, the concentration of copper dissolved in steel currently produced is not more than 0.1% by weight, while ferrous scrap generated from mechanical working of steel or the automobile industry contains copper at a concentration of more than 0.6 wt .%. Normally, the majority, e.g., more than 85%, of copper in ferrous scrap is present as nearly pure metallic copper rather than dissolved elements in steel, in the form of wire for connecting electrical equipments. Namely, the difference in copper concentration between 0.6 wt. % in ferrous scrap and 0.1 wt. % in steel products is attributed to such copper wires mingled with the ferrous scrap.
A number of processes have so far been suggested for the removal of copper from solid or molten ferrous scrap, such as improved physical separation incorporating sorting of scrap, vacuum distillation, sulfide slagging, or the like.
The improved physical separation and sorting of scrap based on human power are able to reduce the copper content in the solid ferrous scrap. However, such techniques are time consuming and labor intensive, hence these are disadvantageous economically. The vacuum distillaion process is based on the fact that copper has a higher vapor pressure than iron. In this process, liquidized ferrous scrap is heated under vacuum for 30 to 60 minutes to evaporate copper. However, the vaporization rate is quite low, hence this process is not feasible for application in large-scale steel production.
J. F. Jordan established the sulfide slagging as a principle process for the removal of copper from liquid ferrous scrap in 1950 (U.S. Pat. No. 2,512,578, June 20, 1950). This process has been the most extensively investigated method. The sulfide slagging is based on the fact that copper sulfide is more stable than iron sulfide at temperatures higher than 600.degree. C. Sodium sulfide is usually added to lower the melting point of iron sulfide and to increase the fluidity. The sulfide slagging can be applied to both solid and liquid ferrous scraps. However, disadvantage is that this process requires sulfides which have not appeared in large-scale industrial production. In addition, the treatment of waste sulfides would not be economical.